Carbon nanotubes as a nanocatalyst and nanoreactor for the efficient treatment of pharmaceutical wastewater via CaSO3 activation

Abstract

Recently, calcium sulfite (CaSO₃) has been increasingly considered to be the most potential substitute for PMS/PDS to produce SO₄·⁻ for degrading target organic pollutants due to its fewer toxic side effects, lower cost, and absence of residues after decontamination. In this regard, herein, we report carbon nanotubes (CNTs) as an efficient catalyst and nanoreactor for activating CaSO₃ for water decontamination. Compared with NaHSO₃ and Na₂SO₃, the result exhibited that the order of tetracycline (TC) removal rate at 30 min as follow: CaSO₃ (80.2%) > Na₂SO₃(49.3%) > NaHSO₃ (48.8%). Because CaSO₃ can slowly discharge SO₃²⁻, it could be used as a sustained-release SO₃²⁻ source, which could effectively solve the quenching issue of SO₄·⁻ at a high concentration of SO₃²⁻. Control experiments confirmed the vital roles of electron transfer and dissolved O₂ in the formation of SO₃·⁻ and SO₄·⁻ in the CNTs/CaSO₃ system, as also verified by quenching experiments and EPR analysis. More importantly, we found that the CNTs/CaSO₃ system had excellent potential for the treatment of actual pharmaceutical wastewater, with the total organic carbon and chemical oxygen demand dropping by 42% and 70% after treatment, respectively. In addition, germination experiments of wheat seeds confirmed that the biotoxicity of TC to wheat seeds was greatly reduced after treatment with the CNTs/CaSO₃ system.